Filters play an important role in many telecommunication systems, such as wireless cellular systems, for example. In one application, bandpass filters are utilized to transmit energy in a desired band of frequencies (i.e., the passband) and to reject energy at unwanted frequencies (i.e., the stopband) that are outside of the desired band or passband. In use, and in a transmit or receive function, multiple bandpass filters may be utilized to divide up the entire receive or transmit band into smaller sub-bands for further processing.
One type of bandpass filter utilizes resonators, such as cavity resonators, that are cascaded together to form a multi-pole filter. Such resonator filters, and their characteristics, are often indicated by a quality factor or Q rating. Since the characteristics of a single filter can have a significant impact on the overall performance of the larger communication system, it is desirable to achieve the most ideal response possible in the filter. One of the major performance limitations is the unloaded Q factor of the resonators.
In addition to maintaining a desirable passband and significant rejection at the stopband, one other performance criterion that is important within a bandpass resonator filter is the amount of bandpass ripple or the loss variation in the filtered signal. Bandpass ripple or loss variation refers to the situation where the filter has more insertion loss at the band edges of the passband than it has at the band center or center frequency of the passband. While a theoretical resonator filter might have resonators with infinite Q, in constructing such resonators and implementing them into real filter applications, they have a finite Q. Filters using resonators of finite, uniform unloaded Q have a certain amount of passband ripple that needs to be reduced to meet desirable system requirements.
One technique for addressing such passband ripple is to utilize predistorted Q in the filter. Predistorted Q refers to a filter design technique wherein the resonator Q is not equal or uniform for all the resonators that are used throughout the filter. To realize an equal ripple passband, which is desirable, the filter transmission poles need to be placed in specific locations on the S plane. Finite resonator Q shifts the poles on the real axis, causing ripple distortion, which results in band edge roll-off. Predistorted Q allows the transmission poles to be placed such that their relative positions are generally identical to the infinite Q positions, but with a relative shift on the real axis. The predistorted Q may thus be utilized to realize a flatter passband ripple.
While various predistorted Q techniques are utilized for filter construction, it is still desirable to improve upon such techniques and to provide predistorted Q within a filter using resonators such that the size and the cost of the filter is not significantly high or prohibitive.
It is further desirable to provide a filter configuration that is adaptable to provide a number of different filters with complex filter functions. The complex functions should be realizable while still controlling passband insertion loss as noted. Furthermore, cost savings are a factor for consideration in any such filter design.